Requirements in the system of the fourth generation mobile communication (hereinafter “4G”) include use of maximum 100 MHz bandwidth in DL (i.e. downlink) to realize 1 Gbps or more in stationary environment and 100 Mbps or more in moving environment and use of maximum 40 MHz bandwidth in UL (i.e. uplink) to realize 300 Mbps or more in stationary environment and 20 Mbps or more in moving environment.
Besides the TDD scheme, the FDD (Frequency Division Duplex) scheme also provides a duplex method to realize the above high-rate transmission, and these schemes can be compared as follows. In the TDD scheme, DL transmission and UL transmission are switched in the same frequency band according to time, so that guard time is necessary to prevent interference between DL transmission and UL transmission. By contrast with this, with the FDD scheme, DL transmission and UL transmission are carried out in different frequency bands, and therefore guard time is not necessary to prevent interference between DL transmission and UL transmission. The FDD scheme is superior to the TDD scheme with regards to adjacent channel interference and latency.
On the other hand, with the TDD scheme, the same device is used for transmission and reception, so that the cost of hardware is cheaper than in the FDD scheme. Further, with the FDD scheme, the DL channel or UL channel to allocate to each UE (i.e. User Equipment) is fixed per UE. However, with the TDD scheme, it is possible to allocate the DL channel and UL channel dynamically per UE according to the amount of traffic. Further, with the TDD scheme, DL transmission and UL transmission are carried out in the same frequency band, that is, the DL band and the UL band are symmetrical in the TDD scheme, so that, in the TDD MIMO (Multi-Input-Multi-Output) system, it is possible to estimate the DL-CSI (Down Link-Channel State Information) that is necessary for DL closed-loop control, in the base station (BS) side without feedback from the UE. Here, “DL-CSI” refers to information about the amplitude fluctuation and phase fluctuation, which a signal has influence from a channel path when the base station transmits a signal to a terminal and is used for DL closed-loop control processing. To be more specific, the base station apparatus performs UL channel sounding using a UL pilot from the UE, based on the channel reciprocity between DL and UL, to estimate UL-CSI, and, assigns the estimated UL-CSI as DL-CSI for DL closed-loop control. In this way, with the TDD scheme, DL closed-loop control can be realized while suppressing the amount of feedback from the UE, so that it is possible to realize improved system throughput and transmission rates.
FIG. 1 shows how the DL channel and UL channel are allocated to each UE symmetrically in a TDD communication system.
FIG. 1 shows a case as an example where the DL bandwidth and the UL bandwidth are 20 MHz alike, and where the DL channel and UL channel are allocated to five UEs, that is, allocated to UE 1 to UE 5. As shown in FIG. 1, in a TDD communication system, the DL channel and UL channel are the same between UEs, that is, symmetrical. In a TDD communication system, DL transmission and UL transmission are switched in time and guard time is provided so that DL transmission and UL transmission do not interfere transmission each other. Based on the reciprocity between DL-CSI and UL-CSI, when the base station apparatus performs UL channel sounding using a received UL pilot, DL-CSI can be acquired. According to this channel sounding, it is possible to perform DL closed-loop control including transmission power control and beam forming without feeding back DL channel state information (see Non-Patent Document 1).
Non-patent Document 1: 3GPP RAN WG1 #44 Denver, USA, Feb. 13-17, 2006 “Uplink sounding for obtaining MIMO channel information at Node B in E-UTRA 13.1.2.”